This invention relates generally to chart recorder systems where a print head is utilized to record data on moving chart paper and, more particularly, to an improved apparatus for maintaining the proper pressure between the print head and the moving chart paper. Chart recorders are, of course, well-known, and one common use of chart recorders is in connection with densitometers of the type disclosed in U.S. Pat. No. 4,005,434 assigned to the Assignee of the present invention.
The data recorded on the moving chart paper may be in various forms such as analog, numeric, and alpha-numeric. In addition, there are various techniques for actually "printing" the data on the moving chart paper such as thermal printer heads, which require a thermo-sensitive chart paper, and ink printing heads. The thermal printing heads, of course, may use either the dot-matrix or line-segment printing technique, both of which are conventional.
One of the basic problems with chart recording devices is the need to maintain constant and predetermined pressure between the printer head or pen head and the moving chart recorder paper. When ink type printing heads or ink pens are used, if the pressure is insufficient to maintain contact between the pen and the chart paper, the output from the pen may skip or may be so light as to be difficult to read. If the pressure is too great, the movement of the chart paper may be impeded or a smudging of the ink on the paper may occur. Furthermore, if the pressure is not constant, the readability of the data on the chart paper will be poor.
Similar problems arise when a line segment or dot-matrix print head is used in conjunction with thermo-sensitive paper. The basic structure for dot-matrix print heads includes an array or series of separate heat output elements on the print head which are selectively excited by electrical signals transmitted to the print head. When a particular element is excited or heated, the transfer of heat from that element to the chart paper causes a visible darkening of the paper to occur.
Insufficient pressure between the print head and the moving chart paper can create a weak, barely visible darkening, or no darkening at all, while too much pressure may impede the movement of the paper or create a large dark area on the paper.
Therefore, it can be seen that the pressure between the print head and the chart paper is particularly important not only in conjunction with ink pen printers but also in conjunction with dot-matrix print heads.
Prior art treatment of the problem of pressure control between the print head and the chart paper centers around the use of a series of coil springs which are used in various configurations to hold the print head in contact with the paper. Whether the coil springs are located vertically or horizontally between the head and a support frame the same difficulties arise. These difficulties included a wearing out or fatiguing of the coil springs either in part or in total and either individually or as a group due to frequent paper changing. That is, when a roll of chart paper is removed from the recorder, the compression of the coil springs is released, and when a new roll of chart paper is inserted, the coil springs are once again compressed to hold the print head against the chart paper. This constant chart paper changing causes uneven wear or fatigue of the springs, thereby causing a change in the amount of pressure between the chart paper and the print head.
Another problem with the coil spring arrangement of the prior art is the difficulty in adjusting the pressure. That is, there is no convenient manner of adjusting the pressure except by changing the number of coil springs or the strength of each coil spring. This becomes particularly relevant when the chart paper being used is of a different sensitivity than that of the prior chart paper. The pressure between the chart paper and the print head cannot be changed without great difficulty as it requires some adjustment of the individual coils or changes in the structure of the coils.
Yet another structural difficulty with the coil spring arrangements of the prior art is the manner in which the frames upon which the coil springs are mounted interfere with access to the chart recorder and paper. The mounting support structures occupy a significant portion of the open area around the chart paper and print head thereby restricting access to the pens or print heads when adjustment, inspection or replacement of the pens or print head is necessary.
The problems of the prior art print head biasing devices are overcome by the present invention wherein the pressure remains constant over a long period of use and may be adjusted with ease.